This research will attempt to pinpoint an apparent inhibitory effect caused by changing the sterol found in Tetrahymena membranes on the activity of a fatty acid desaturase. Tetrahymanol can be completely replaced in membranes in vivo by other sterols such as ergosterol. The enzyme system apparently affected is that which desaturates oleic acid to linoleic acid in this organism, while another desaturase system, that which introduces double bonds at C-6, is not inhibited. It is proposed that this differential effect represents a regulatory mechanism whereby Tetrahymena can adjust the quantities of polyunsaturated fatty acids in their membrane phospholipids so as to maintain optimum membrane fluidity. It is hoped to gain more knowledge about the sophisticated regulatory mechanisms governing membrane assembly by studying this apparent effect of sterol substitution on fatty acid biosynthesis. These results may apply to the role of sterols, including cholesterol, in other organisms. Desaturase activity is assayed using radioactive substrates, either in vivo or in vitro, with subsequent isolation and analysis of products. This research will study the properties of the desaturase systems mentioned, also will attempt to discover the exact nature of the sterol inhibitory effect on the oleoyl CoA desaturase. It will be necessary to eliminate the possibility that this effect is an indirect one, which results from a primary effect on another enzyme or enzyme system. An example might be the acyltransferase(s) which places the desaturation product into phospholipids, whose inhibition might then lead to accumulation of product and inhibition of the desaturase system. Where possible, studies will be carried out on microsomal preparations. Preliminary results indicate that soluble regulatory factors may be involved. The desaturation of linoleate at C-6 has been measured in vivo, but as yet not been observed in vitro. Some development of new techniques to study this activiiy is required.